In the case of the iPhone, the search for the current carrier network is usually automatic. Sometimes, however, you may need to manually select a carrier. This kind of situation is generally when there is no signal when going abroad, because the iPhone generally searches the 3G network first, and then the GSM network. If there is a problem with the 3G network at this time, it will lead to no signal for a long time. Here's how to manually select a carrier on an Apple iPhone.
iPhone
Start by opening the "Settings" app on the home screen, as shown in the picture
Find the "Carrier" section in the general list and click to open it.
Here you can see that the system automatically selects the operator by default, click to turn off the automatic switch.
Then you will see that the system is searching for signals from the carrier network around it, as shown in the figure
Please wait for a moment, the list of supported carriers will appear, click to select.
It is worth noting that if you choose the wrong carrier, it will lead to no service, you are a card, but choose the mobile network.